The invention relates to thermal-radiation imaging devices of the type having a detector element comprising a strip of semiconductor material in which free charge carriers can be generated on absorption of thermal radiation and in which an ambipolar drift of radiation-generated minority charge carriers can occur along the strip. The invention further relates to thermal-radiation imaging systems comprising such a device.
British Pat. No. 1,488,258 discloses a thermal-radiation imaging device comprising a strip of semiconductor material in which free charge carriers can be generated on absorption of thermal radiation incident on the strip. Biasing-electrode means are spaced in a direction along the strip for causing a bias current predominantly of majority charge carriers to flow along the strip. The bias current is capable of supporting an ambipolar drift of radiation-generated free minority charge carriers in the opposite direction to the bias current. A readout area is present in a part of the ambipolar drift path between a pair of spaced electrodes.
The semiconductor material of the strip is usually n-type cadmium mercury telluride. One of the two electrodes of the pair associated with the readout area is usually common with a biasing electrode. These electrodes may be of a metal such as aluminium which forms an ohmic contact to n-type cadmium mercury telluride. In the known device these readout electrodes extend across the width of the strip. The voltage developed between the pair of electrodes in use of the device is a measure of the density of minority carriers generated by the radiation and corresponding to successive elements of the thermal image which is scanned along the strip.
The spacing between the two read-out electrodes defines a localized sample area over which the ambipolar charge carrier density is averaged. The electrical signal derived from these electrodes is a measure of this averaged density. In order to obtain good image resolution from the device, it is desirable for the pair of readout electrodes to have a close spacing. However, reducing this spacing reduces the resistance and minority-carrier transit time through the readout area. This reduces the noise and responsivity of the detector element, and may result in the detector output signal being degraded by the characteristics of an amplifier or other circuit connected to the read-out electrodes.